I don't add tools to my toolbox very often. I tend to find that the time spent learning them outweighs the extra benefit I might obtain. Recently I found (or was pointed at) a couple of nice ones, though.
First is ack a nice grep replacement. Among other things it highlights what you're searching for, is automatically recursive, and knows how to ignore .svn and similar directories. Quite cute. It looks like it has a million bells and whistles, but it usually just Does the Right Thing (tm). "ack foo" usually gives me exactly what I want. So I just went looking for "CreateFunctionStmt" in the Postgres sources, and the output was really quite nice - much nicer than "grep -r" would have given me.
Second is Trang, a nifty tool to convert between the various languages for describing XML languages. So you can take a DTD and get an XML Schema or RELAXNG spec from it, for example. I used it to convert the (out of date) RELAXNG spec we had for the new XML explain output in Postgres into an XML Schema, and it worked perfectly. It will only output XML Schemas, it won't take them as input, but that still met my needs. It can also infer a schema from an example XML document, which is very nice indeed. There are limitations, to be sure, but it certainly beats starting from scratch, so it's a very definite time saver.